Jamieson, Fausset and Brown on Galatians 3:6

6. The answer to the question in Ga 3:5 is here taken for granted, It was by the hearing of faith: following this up, he says, "Even as Abraham believed," &c. (Ge 15:4-6; Ro 4:3). God supplies unto you the Spirit as the result of faith, not works, just as Abraham obtained justification by faith, not by works (Ga 3:6, 8, 16; Ga 4:22, 26, 28). Where justification is, there the Spirit is, so that if the former comes by faith, the latter must also.